1. Field of the Disclosure
The present disclosure relates to Radio Frequency (RF) Power Amplifier (PA) systems, and more specifically to an RF PA system with reduced spectral noise.
2. Description of the Related Art
RF transmitters and RF PAs are widely used in portable electronic devices such as cellular phones, laptop computers, and other electronic devices. RF transmitters and RF PAs are used in these devices to amplify and transmit the RF signals remotely. RF PAs are one of the most significant sources of power consumption in these electronic devices, and their efficiency has a significant impact on the battery life of these portable electronic devices. For example, cellular telephone makers make great efforts to increase the efficiency of the RF PA systems, because the efficiency of the RF PAs is one of the most critical factors determining the battery life of the cellular telephone and its talk time. One approach to increasing the efficiency of RF PA systems is envelope tracking. In envelope tracking, the power supply voltage applied to the PA is constantly adjusted to ensure that the PA is operating at peak efficiency for the given instantaneous output power requirements.
Another concern in RF PAs is keeping the distortion in the RF output signal of the RF PA below acceptable levels. Distortion occurs whenever the actual RF output signal of the RF PA differs from the desired RF output signal. Amplitude differences, phase differences, and other factors can contribute to the distortion in the RF output signal. Envelope tracking power systems attempt to minimize distortion by adjusting the supply voltage to the PA so that the gain of the PA is as uniform as the output power requirements change. Having a uniform gain in the PA tends to minimize amplitude and phase errors in the output signal. However, attempting to maintain a uniform gain in the PA can introduce non-monotonic variations in the supply voltage to the PA that increase spectral noise produced by the RF PA. Since the RF spectrum is shared amongst users of cellular telephones, having a wide spectral distortion is undesirable.